The present invention relates to an active nutation damping control system for dual-spin stabilized devices or bodies in which a sensor senses nutation directly and damps it by activation of the despin motor.
The invention is useful with any dual-spin stabilized device or body whether it is of a stable configuration or not. By dual-spin stabilized device is meant a stabilized device having a spinning portion or rotor, and a despun portion or platform, the two portions being coupled together by a despin motor and bearing assembly. In an unstable dual-spin stabilized device, the spin moment of inertia of the rotor in relation to the combined transverse moment of inertia of the device is less than one or unity, and in a stable configuration the spin moment of inertia of the rotor in relation to the combined transverse moment of inertia of the device is greater than one or unity.
The invention is useful with spin-stabilized devices or vehicles operable in differing environments in differing operational modes, such as a fluid-supported mode of operation as by means of an air bearing, a zero gravity mode of operation or other defined or random mode of operation. Devices or vehicles of this general character are employed in applications ranging from the laboratory to outer space. In the laboratory such devices or vehicles are usefully employed as environmental test beds, duplicating the essential modes of environmental operation, including force suspension which provides spatial freedom, providing a basis for testing or proving spin stability, attitude control and the performance of instrumentation, etc. In operation in space, such devices or vehicles are useful as communication links and as scientific devices for gathering and transmitting information as to physical conditions.
A spin-stabilized body exhibits certain types of troublesome motions called wobble, precession, or nutation. All such motions tend to result in a displacement of the body's geometric axis from its intended mission orientation or attitude. Nutation of a satellite, or the coning motion of the bearing or spin axis about the total angular momentum vector, may result from any of the following disturbances: (1) booster final stage angular motion, (2) operation of the separation equipment, (3) bombardment by micrometeorites, (4) operation of payload components with uncompensated momentum, and (5) operation of mass expulsion devices on the spin stabilized devices.
In general, nutation may be reduced by energy absorbing or momentum transfer devices operable on either or both of the transverse axes to attenuate the nutation.
In one type of prior art system, nutation is damped by means of the despin control system. The despin control system operates to control the despin motor to keep the platform stably pointed in a predetermined direction, and because nutation introduces an attitude error signal, the despin control system will also damp the nutation. However, in this system the attitude sensors only weakly sense nutation as a second-order effect, and the primary design of the system must satisfy the despin requirements having to do with the pointing of the platform toward a particular direction, rather than satisfying nutation damping requirements.
A second type of system provides active nutation damping by appropriately phased thruster pulses. Such a system is shown in the paper entitled, "An On-Board, Close-Loop, Nutation Control System for a Spin-Stabilized Spacecraft," by Lynn H. Grasshoff, published in the May 1968 edition of The Journal of Spacecraft and Rockets, Volume 5, No. 5. This prior art system has the disadvantage that its operation results in attitude (angular momentum) perturbation and propellant consumption.
A third type of prior art system employs passive nutation damping by platform-mounted nutation dampers such as eddy current dampers having a pendulum. Such passive dampers have a damping performance and linear range of operation that can degrade significantly with platform spin rate or off nominal rotor spin rates. As the nutation increases to a certain point, the pendulum hits the stops and the damper is ineffective.
Accordingly, it is an object of the present invention to provide a nutation damping system which takes maximal advantage of the platform dynamic imbalance to damp nutation while only weakly coupling the despin attitude or pointing function with the nutation control function.
Another object of the invention is to provide a nutation damping system that is self-tuning, in that it can apply optimally phased torque over a wide range of platform spin rates or off nominal rotor spin rates.
A further object of the invention is to provide a nutation damping system whose performance degrades gradually and predictably under saturated conditions.
A still further object of the invention is to provide a nutation damping system whose weight is significantly less than a passive nutation damper for similar performance.
An even further object of the present invention is the provision of a nutation damping system that does not depend upon the use of thrusters so that it requires no propellant and does not perturb the attitude of the spinning body.